a) Field of the Invention
The invention is directed to a length measurement device for measuring the relative position of two objects in which a scale is connected with one object via retaining elements and the ruling of the scale is sensed by a sensing unit formed of a measurement head and a measurement carriage, wherein the sensing unit is coupled to the other object via a coupling point by means of drivers which are constructed as a deflectable, elastic coupling device. Such length measurement devices are used, for example, as combination positioning and measuring devices in machine tool construction and in mechanical engineering in general.
b) Description of the Related Art
Numerous measurement devices in which the relative movements of two mechanical structural component parts, for instance, are measured with high accuracy are known from the prior art. Strict requirements are imposed on the guidance of the sensing unit as regards positional tolerances as well as torsional or rotational tolerances. In machine tool construction, machine guides do not meet these strict requirements, so that the sensing unit is normally accommodated in a special measurement carriage which is guided on an auxiliary guide within the measurement system and is oriented to the scale. In order to balance or compensate the guide paths of the two guides relative to one another, coupling members are arranged in the measuring direction between the driver, which is securely fastened to the first machine element, and the measurement carriage which runs on the auxiliary guide in the measurement system.
DE-PS 39 08 260 discloses an arrangement in which a ball is located between a plane surface at the measurement carriage and another plane surface at the driver, this ball being held in contact with the contact faces by means of a spring. The frictionally engaging coupling compelled by the spring requires high spring forces on the one hand and, on the other hand, hardened contact faces between the ball and driver and between the ball and measurement carriage in order to reduce Hertzian flattening. Beyond this, the manufacturing and adjustment requirements for aligning the contacting surface extending vertical to the measurement direction and for keeping within the measured value error limits are relatively elaborate.
DE-PS 28 10 341 describes another type of length measurement device in which a coupling element in the form of a wire is arranged between the driver and the measurement carriage. The coupling element is clamped by one end at the driver and by the other end at the measurement carriage. When compensating for guidance errors, the coupling point moves in an approximately circular path with the radius of the coupling wire length. The resulting measurement errors depend to a decisive extent upon the position of the coupling point corresponding to the attaching or mounting tolerances of the measurement system at a machine part.
The longitudinal stiffness of the coupling element drops sharply in the case of large deflections, which results in measurement errors, particularly with large dynamic loads and with reversals in the measurement direction.